There are a variety of types of sensors using a movable mass(moving mass). For example, there is an inertial sensor such as an accelerometer and an angular accelerometer (vibration gyroscope).
An accelerometer, which detects an acceleration of a vehicle, generally uses a piezoresistive effect. According to such a sensor, for example, a box shape of seismic mass (i.e. moving mass) is contained in a cavity of a silicon base member (fixed frame). The movable mass is suspended by beams on which piezoresistors are formed, so that a stress is applied to the piezoresistors in response to movement of the mass. The variation of stress applied to the piezoresistors is detected as a variation of resistance. Such technology can be used for cruise control of vehicles.
The above-described mass is required to move freely, however, if the mass over moves, the sensor may be broken or damaged. According to an invention shown in Japanese Patent Publication Kokoku H5-71448, a glass stopper is used for inhibiting over-move of the mass.
However, because the glass stopper is arranged above the mass, the thickness of the sensor is increased, and the fabrication process is more complicated. Thus fabrication costs are increased. Further, stress generated between the glass and silicon member may negatively affect the characteristics of the sensor.